1. Field of the Invention.
This invention relates to packaging techniques for integrated circuits and, more particularly, to techniques for sealing an integrated-circuit package.
2. Prior Art
A hermetically-sealed package, or package assembly for an integrated circuit typically includes a base in which is formed a cavity for the integrated-circuit die. The base of the package is formed of a ceramic or a metallic material. A lid of ceramic or metallic material is sealed to the base with an appropriate layer of sealing material, such as glass, solder, or epoxy. Sometimes an intermediate, thin sealing ring is positioned between the base and the lid so that two layers of sealing material are used, one on each side of the sealing ring. Currently, all of the sealing techniques used in packaging of integrated circuits rely upon the adhesion strength of the layer of sealing material. Because of the differences in the temperature coefficients of expansion (TCE) of the various materials used in a package, the adhesion of the sealing material fails and the lids of the packages peel away from their bases. This occurs particularly when a package is subjected to pressure-pot testing or to thermal-cycle testing, where the shear forces cause the sealing materials to fail at the margins between the sealing materials and the package elements.
With variations in temperature, the various components of a package, as well as the sealing layers, expand or contract at different rates so that shear forces are generated at the interfaces between the different elements and the sealing materials. The shear strengths of the sealing materials, such as glass, solder, or epoxy are relatively weak so that failures occur at the margin of the sealing material near the package elements. Typically, an epoxy material fails with a shear pressure of 1 to 5 kpsi. The materials of the package elements such as the base or lid are much stronger under shear loads. For example a metal has a failure shear strength of 10-30 kpsi.
One solution to the problem of lids peeling from packages is to attempt to match the TCE's of the various materials of the package assembly. This is difficult to accomplish and has the disadvantage that more expensive materials often must be used.
The need has arisen for an improved integrated circuit packaging technique which enhances and reinforces currently-used packaging technique by increasing the adhesion strength of adhesive or bonding material between the lid and the base of an integrated-circuit package.